1. Field of the Invention
This invention relates to the field of measuring and testing respiration characteristics of a subject. More particularly, the invention relates to respiration test methods for providing accurate test signals which may be used to measure the duration of a subject's inhalation and exhalation periods, and electronic timing circuits for performing such methods.
2. Description of the Prior Art
Military, law-enforcement, construction, and other personnel often wear respiratory equipment while doing a variety of duties ranging from deploying chemical agents to removing toxic waste to exploring underwater regions. One critical problem confronting designers of such respiratory equipment has been finding ways to increase equipment effectiveness especially during periods when a wearer is doing a strenuous task. Designers have long known that equipment effectiveness can be significantly increased by reducing its physiological burden on the wearer. However, an accurate measurement of the full physiological burden of wearing most respiratory equipment during high-stress periods is often difficult to obtain.
When testing respirators to evaluate their physiological burden on a subject, it has been the general practice to use a flow transducer to measure the volume of expired air and the breathing rate (breaths per minute) of the subject. Designers of respiratory test equipment have recently come to recognize the need to also accurately and reliably measure the actual duration of each of the subject's inhalation and exhalation periods. Most prior art flow measuring systems employed for making such duration measurements have performed satisfactorily when the subject being tested is physically inactive. However, when the subject is active, many prior art systems do not accurately and reliably measure the actual duration of each of the subject's inhalation and exhalation periods.
More specifically, conventional flow transducers used to test respirators typically have a pair of flow meters. One flow meter usually mounts on the inspiratory side of the respirator while the other mounts on the expiratory side. This allows simultaneous recording of the volume flows for both inhalation and exhalation. A commercially available flow meter system that has been successfully used for making such flow measurements is the KL Engineering Kozak Turbine Compensator KTC-3-D. However, this and similar systems have not proved entirely satisfactory for measuring breath duration of an active subject for the reason that synchronization and timing of the inspiratory and expiratory flow transducers are not sufficiently accurate.